Electronic circuits are frequently made by plating copper on an electrically insulating substrate made of a material, such as silicon or gallium arsenide. If the circuit contains elements that generate heat, the heat must be removed or the electronic components will deteriorate or be destroyed. In terrestrial applications, removal of heat can be done by blowing air over the circuit but in space applications, heat is removed only by conduction down through the substrate. The substrate must, therefore, be attached by solder or braze to a metal heat sink or support so that there is a good thermal bond between the heat sink and the substrate. At the same time, the thermal expansion rate of the substrate must approximate that of the heat sink so that the bond between the substrate and the heat sink will not fracture under thermal cycling. Unfortunately, the materials that presently qualify for use as a substrate are ceramics, such as silicon and gallium arsenide, that have a very poor thermal conductance. This causes the electronic devices on the substrate to operate at elevated temperatures which limit the life of the circuit.
Diamond, such as made by chemical vapor deposition (CVD), has been proposed as a superior substrate for thermally stressed electronic circuits and as a heat spreader. Diamond has about five times the thermal conductivity of pure copper and diamond is an electrical insulator. Diamond, as well as ceramics, can also be plated with a metal so that circuits can be created on a large, flat diamond or ceramic sheet. However, the large differential in thermal expansion rates between diamond and metals or ceramics limits the useful size of the diamond substrate to about one square centimeter. Larger diamond substrates tend to fracture due to stresses induced by thermal cycling.